I know exactly what you are saying and I run into that instance in all genre music. My solution is not really changed the genre of the music, I simply tag it in the "grouping" as a gangsta rap along with the year is came out and based on that it will go into that playlist.
For instance, Notorious BIG Big Poppa is in the 1990s and its still under the Hip-Hop/Rap genre and the grouping tag is Chill. based on the year and hte group tag, it will go into the chill hip hop playlist or w/e.
At the end of the day, organize it the way that works for you, I personally find it easier to classify house music over hip-hop rap, etc. You really have to listen to the song, see what the crowd thinks of it and based on that tag it. Some songs can be gangsta and party AND chill. u cant put that in genre but you can tag it as gangsta, chill, and party in the groupings.
I use groupings but ou can use w/e other tags in the mp3 file.